• Geophysics and Geophysical Exploration
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• v.19 no.3
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• pp.136-144
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• 2016

Since seismic inversion is based on the wave equation, it is important to calculate the solution of wave equation exactly. In particular, full waveform inversion would produce reliable results only when the forward modeling is accurately performed because it uses full waveform. When we use finite-difference or finite-element method to solve the wave equation, the convergence of numerical scheme should be guaranteed. Although the general proof of convergence is provided theoretically, the consistency and stability of numerical schemes should be verified for practical applications. The implementation of source function is the most crucial factor for the consistency of modeling schemes. While we have to use the sinc function normalized by grid spacing to correctly describe the Dirac delta function in the finite-difference method, we can simply use the value of basis function, regardless of grid spacing, to implement the Dirac delta function in the finite-element method. If we use frequency-domain wave equation, we need to use a conservative criterion to determine both sampling interval and maximum frequency for the source wavelet generation. In addition, the source wavelet should be attenuated before applying it for modeling in order to make it obey damped wave equation in case of using complex angular frequency. With these conditions satisfied, we can develop reliable inversion algorithms.

• Geophysics and Geophysical Exploration
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• v.19 no.3
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• pp.121-130
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• 2016

Several factors are discussed that should be considered in measuring thermal conductivity of rock cores with a PEDB (potable electronic divided bar) system, which is relatively accurate and easy to operate, and can measure the thermal conductivity of rock cores for various diameters. Then the system is applied to measure thermal conductivity of 70 rock cores from Ulleung Island. Air temperature affects most on the thermal conductivity measurements, so that it is very important to minimize the temperature change during the measurement. Other factors such as the temperature of heat source, averaging time window on the thermal conductivity measurements do not affect much compared to air temperature. Slightly higher thermal conductivity is measured when using the thermal contact paste between the sample and heat source or heat sink. Especially, rock cores with irregular surface showed bigger difference. Repeatability showed less than ${\pm}0.3%$ for standard samples and less than ${\pm}4%$ for rock samples, respectively, when the room temperature changes within $1^{\circ}C$ during the measurements. Thermal conductivity of the rock cores from Ulleung Island roughly increases as depth increases but does not show any dependency on the rock types.

• Geophysics and Geophysical Exploration
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• v.19 no.3
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• pp.111-120
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• 2016

In order to enhance the connectivity of fracture network as fluid path in enhanced/engineered geothermal system (EGS), the exact locating of hydraulic fractured zone is very important. Hydraulic fractures can be tracked by locating of microseismic events which are occurred during hydraulic fracture stimulation at each stage. However, since the subsurface velocity is changed due to hydraulic fracturing at each stage, in order to find out the exact location of microseismic events, we have to consider the velocity change due to hydraulic fracturing at previous stage when we perform the mapping of microseimic events at the next stage. In this study, we have modified 3D locating algorithm of microseismic data which was developed by Kim et al. (2015) and have developed 3D velocity update algorithm using occurred microseismic data. Eikonal equation which can efficiently calculate traveltime for complex velocity model at anywhere without shadow zone is used as forward engine in our inversion. Computational cost is dramatically reduced by using Fresnel volume approach to construct Jacobian matrix in velocity inversion. Through the numerical test which simulates the geothermal survey geometry, we demonstrated that the initial velocity model was updated by using microseismic data. In addition, we confirmed that relocation results of microseismic events by using updated velocity model became closer to true locations.

• Geophysics and Geophysical Exploration
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• v.18 no.1
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• pp.31-34
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• 2015

A gravity meter has been used for exploring subsurface mineral resources and monitoring long-period events such as Earth tides. Recently, researchers found several other intriguing features that we could even detect large teleseismic earthquakes and monitor seismic ambient noise using gravimeters. The zero-length spring suspension technology gives the gPhone (Micro-g LaCoste) excellent low frequency sensitivity, which may have implications for investigating much longer-period natural events (e.g., Earth's hum, tsunami waves, etc.). In this study, we present preliminary results through temporary operation of the gPhone at Geumsan in South Korea for 9 months (Nov. 2008-Jul. 2009). The gPhone successfully recorded large teleseismic events and showed a clear seasonal variation of the Double frequency microseisms during its operation period.

• Geophysics and Geophysical Exploration
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• v.19 no.2
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• pp.76-83
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• 2016

Migration image can be distorted due to reflected waves in the source and receiver wavefields when discontinuities of input velocity model exist in seismic imaging. To remove reflected waves coming from layer interfaces, it is a common practice to smooth the velocity model for migration. If the velocity model is smoothed, however, the subsurface image can be distorted because the velocity changes around interfaces. In this paper, we attempt to minimize the distortion by reducing reflection energy in the source and receiver wavefields through acoustic impedance homogenization. To make acoustic impedance constant, we define fake density model and use it for migration. When the acoustic impedance is constant over all layers, the reflection coefficient at normal incidence becomes zero and the minimized reflection energy results in the improvement of migration result. To verify our algorithm, we implement the reverse-time migration using cell-based finite-difference method. Through numerical examples, we can note that the migration image is improved at the layer interfaces with high velocity contrast, and it shows the marked improvement particularly in the shallow part.

• Geophysics and Geophysical Exploration
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• v.13 no.4
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• pp.336-348
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• 2010

The Single-channel seismic survey with the source of bubble pulser and drilling survey was carried out in 2008 and 2009 for the site survey of Daecheon-Wonsando area, which was a proposed area of Korea-China subsea tunnel. The goal of this study is to analyze the depth and characteristics of acoustic basement for the stability assessment and tunnel design in this proposed area through combining drilling data with this single-channel seismic data after detailed processing. For this purpose, among the data processing schemes which are usually applied to multi-channel seismic data, we applied the F-K filtering to eliminate the AC(alternating current) noise and the post-stack depth migration to produce depth section. As a result, we verified that the improved depth section could be obtained from single-channel seismic data, and the distribution and characteristics of basement could be analyzed in survey area through the combined analysis with drilling data. However, we could not interpret the detailed structures, fault and fracture zone, due to the quality of bubble pulser source and single-channel data. We expect that those detailed structures can be analyzed when higher resolution seismic data is provided. Therefore, we recommend some items for future seismic survey of subsea tunnel to obtain the high resolution seismic data.

• Geophysics and Geophysical Exploration
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• v.13 no.4
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• pp.323-329
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• 2010

Three deconvolution methods were applied to stacked seismic data obtained to investigate gas-hydrates in the Ulleung Basin, East Sea: (1) minimum-phase spiking deconvolution, (2) minimum-phase spiking deconvolution using an averaged wavelet from all traces, and (3) deterministic deconvolution using a wavelet with phases computed from well-logs. We analyzed the resolving property of these methods for lithological boundaries. The first deconvolution method increases temporal resolution but decreases lateral continuity. The second method shows, in an overall sense, similar results to the spiking deconvolution using a minimum phase wavelet for each trace; however, it results in a more consistent and continuous bottom-simulating reflector (BSR) and better resolved sub-BSR reflectors. The results from the third method reveal more detailed internal structures of debris-flow deposits and increased continuity of reflectors; in addition, the seafloor reflection and the BSR appear to have changed to a zero-phase waveform. These properties help more precisely estimate the distribution and reserves of gas hydrates in the exploration area by improving analysis of facies and amplitude of the BSR.

• Geophysics and Geophysical Exploration
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• v.13 no.4
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• pp.315-322
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• 2010

The phase-screen and the split-step Fourier migrations, which are implemented in both the frequency-wavenumber and frequency-space domains by using one-way scalar wave equation, allow imaging in laterally heterogeneous media with less computing time and efficiency. The generalized-screen migration employs the series expansion of the exponential, unlike the phase-screen and the split-step Fourier migrations which assume the vertical propagation in frequency-wavenumber domain. In addition, since the generalized-screen migration generalizes the series expansion of the vertical slowness, it can utilize higher-order terms of that series expansion. As a result, the generalized-screen migration has higher accuracy in computing the propagation with wide angles than the phase-screen and split-step Fourier migrations for media with large and rapid lateral velocity variations. In this study, we developed a 2D prestack generalized-screen migration module for imaging a complex subsurface efficiently, which includes various dips and large lateral variations. We compared the generalized-screen propagator with the phase-screen propagator for a constant perturbation model and the SEG/EAGE salt dome model. The generalized-screen propagator was more accurate than the phase-screen propagator in computing the propagation with wide angles. Furthermore, the more the higher-order terms were added for the generalized-screen propagator, the more the accuracy was increased. Finally, we compared the results of the generalizedscreen migration with those of the phase-screen migration for a model which included various dips and large lateral velocity variations and the synthetic data of the SEG/EAGE salt dome model. In the generalized-screen migration section, reflectors were positioned more accurately than in the phase-screen migration section.

• Geophysics and Geophysical Exploration
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• v.13 no.4
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• pp.307-314
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• 2010

Although conventional seismic data processing is based on the assumption that the media are isotropic, the subsurface is often anisotropy in shale formation or carbonate with cracks and fractures. This paper presents the anisotropic parameter and seismic modeling in transversely isotropic media with a vertical symmetry axis using seismic physical modeling. The experiment was successfully carried out with VTI media, laminated bakelite material, using contact transducer of p and s-wave transmission. The variation of velocities with angle of incidence was clearly shown in anisotropic material. Comparing these velocities with the calculated phase velocities, the (P) and (S)-wave velocity observed in anisotropic material was a very good agreement with the calculated values. Anisotropic parameter ${\varepsilon}$, ${\delta}$, ${\gamma}$ was estimated by using Lame's constant calculated from the observed velocity. For the purpose of testing (S)-wave polarization, a birefringence experiment was carried out. The higher velocity was associated with the polarization parallel to the fracture, and the lower velocity was associated with the polarization perpendicular to the fracture.

• Geophysics and Geophysical Exploration
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• v.13 no.4
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• pp.407-415
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• 2010

MT soundings were carried out in 2008, in northern Victoria, Australia, as a continuing collaboration research of 2007 between Republic of Korea, Australia, and Japan. The main purpose of this research is to investigate electrical conductivity structure and thus help understanding of tectonic structure in central Victoria, which is believed to be closely linked to mineralization and magmatic processes of this region. The survey area is located in western Lachlan Fold Belts, which is the part of Tasman Fold Belts in southeastern Australia. An MT profile of 2008 is almost parallel to the one of 2007 and approximately 50 km away. The 2D inversion result of MT data also shows that the position of conductivity discontinuity near surface are well matched with the positions of major faults, such as Avoca Fault, which is the structural boundary between Stawell and Bendigo Zones, and Heathcote Fault Zone, which marks the boundary between Bendigo and Melbourne Zones. It is also confirmed from resistivity image that internal faults in Bendigo Zone are in listric form, which is implied to be formed by structural shortening during compressional orogenic activity in Silurian.